The present invention relates in general to the measurement of various parameters or characteristics of a web of sheet material as it is being manufactured and, more particularly, to a method and apparatus for compensating for deflections within instruments measuring web characteristics by means of a focused radiation beam, such as a focused beam of light.
Many on-line instruments use focused radiation beams to measure characteristics of webs of sheet material, such as paper products, as the webs are being manufactured. Examples of characteristics measured using focused radiation beams include color, gloss, brightness, surface moisture, coating and smoothness. When the position of a web of product, referred to as "process" in the industry, deviates from its intended passline or moves relative to a focused beam instrument, measurement errors will typically result. Passline deviation error problems are normally addressed by means of guides which are attached to the instrument to constrain the process while the instrument is scanned over the product.
Unfortunately, in many applications, it is impractical to guide the web or process from the instrument side. In some applications it is not practical to contact the process or product damage occurs. In other applications, the process is measured using instruments located on both of its sides. If each instrument constrains the process, the process is "pinched" between two guides located on its opposite sides.
An example of the process pinching problem is color measurement of paper webs wherein the paper web or process may be backed by a tile of known color. A backing tile is contained in a module on the side of the process opposite to the instrument. For proper calibration of the instrument, the distance between the backing tile and the process must be controlled. However, the process also needs to be maintained at the proper focal distance from the instrument. Thus, the process should be constrained from both sides which would result in pinching the process.
If the process is restrained on only one side, for example on the backing tile side for a paper web color measurement, the color measurement instrument on the opposite side of the process suffers errors resulting from deflection of a scanning frame which typically supports the instrument.
Some instruments transmit a focused radiation beam through a web or product to a reflector located on the opposite side of the web. If the process is restrained on either the transmitter or the reflector side, errors can result when the process moves out of the focal plane of the measuring beam when the scanning frame deflects.
The response of an optical instrument using a focused radiation beam as a function of misalignment or improper focus of the beam on a web of product are typically characterized by a nearly parabolic curve. The instrument or sensor response is maximum when the process is located at the expected passline, i.e. the radiation beam is accurately focused on the process, thus defining the vertex of the parabola. As the instrument or web of product move relative to one another such that the radiation beam is no longer focused on the web, the response of the instrument decreases along the parabolic curve.
If the process is constrained, using a Bernoulli effect hold down device on the reflector or backing tile side of the process, an error in measurement will occur due to deflection between the instrument and the process. If the absolute distance is measured between the instrument and the process or the hold down device, a correction factor can be derived from the instrument response curve, such as the parabolic curve referred to above.
Measurement of absolute distance between the instrument and the process or hold down device for a focused beam measuring device could thus be a solution to the deflection error. Unfortunately, absolute distance measurements having sufficient accuracy, temperature stability and long term stability, which could serve as a compensating measurement are not practical. Devices like optical triangulators, eddy current or magnetic distance sensors can very accurately sense change in distance to the process or hold down device. However, the absolute distance indicated by such instruments may vary with time, temperature or angular alignment.
Accordingly, there is a need for an improved arrangement for compensating for deflection in measuring instruments utilizing a focused radiation beam. Ideally, the arrangement would be inexpensive and simple to implement facilitating its inclusion into new instruments and its retrofitting into many existing instruments.